Performing scenario reduction using a dominance relation on a set of corners

ABSTRACT

Some embodiments of the present invention provide techniques and systems for performing scenario reduction using a dominance relation on a set of corners. During operation, the system can receive a design library which specifies gate characteristics at each corner in a set of corners. Next, the system can use the design library to determine a dominance relation on the set of corners for each gate type. The dominance relations can be stored with the design library. The system can then receive a set of scenarios over which a circuit design is to be optimized. Next, the system can determine a subset of the set of scenarios using one or more dominance relations on the set of corners. The system can then optimize the circuit design over the subset of the set of scenarios.

TECHNICAL FIELD

This disclosure relates to electronic design automation (EDA). Morespecifically, this disclosure relates to methods and apparatuses forperforming scenario reduction using a dominance relation on a set ofcorners.

BACKGROUND Related Art

The relentless miniaturization and increasing complexity of integratedcircuits have been key driving forces behind the rapid advances incomputer technology.

The advance to smaller semiconductor geometries has increased thevariability in process and operating conditions of integrated circuits.A process corner is a particular set of process conditions under whichthe circuit design may be manufactured. An operating corner is aparticular set of operating conditions (e.g., temperature, voltage)under which the circuit design may operate. The term “corner” can referto a process corner, an operating corner, or a combination thereof.

One reason for the increase in complexity of circuit designs is theintroduction of a large number of operational modes (e.g., stand-by,mission, test, etc.). Different operational modes (or “modes” for short)can have drastically different voltages and clock speeds. In fact, someparts of the circuit design may be completely switched off in certainmodes.

Note that modes are different from corners. Specifically, modes aredesign dependent, i.e., the circuit design dictates the different modesin which a circuit design can operate. For example, typical modes for acircuit design include, but are not limited to, a normal mode, a sleepmode, a stand-by mode, and a test mode. On the other hand, a cornerdepends on the ambient conditions that are expected to occur duringmanufacturing and/or operation. For example, if a chip is expected to beused over a temperature range of −40° C. to 80° C., then the circuitdesigner may define multiple corners that are associated with differentoperating temperatures.

An important goal is to ensure that a circuit design meets thefunctional and performance requirements under all conditions that areexpected to occur. The term “Multi-Corner/Multi-Mode (MCMM) scenario,”or “scenario” for short, refers to a combination of a corner and a mode.

Circuit optimization is a complex task that can consume a significantportion of the circuit design time. Therefore, the task of optimizing acircuit design across multiple scenarios can easily becomecomputationally intensive if the number of scenarios is large.

Unfortunately, the number of scenarios over which circuit designs aredesired to be optimized is increasing rapidly as semiconductorgeometries become smaller and circuit designs become more complex.

SUMMARY

Some embodiments of the present invention provide techniques and systemsfor reducing the number of scenarios over which a circuit design isoptimized. During optimization, the circuit design is modified so thatone or more design constraints are met over a set of scenarios, whereineach scenario in the set of scenarios can be associated with a cornerand a mode. A corner can be associated with an operating temperature anda power supply voltage. The set of modes can include, but is not limitedto, a normal mode, a sleep mode, a standby mode, and a test mode.

In some embodiments, the system can receive a set of scenarios overwhich the circuit design is to be optimized. Next, the system candetermine a subset of the set of scenarios using a dominance relation onthe set of corners such that if the circuit design meets a set of designconstraints in each scenario in the subset of the set of scenarios, thecircuit design is expected to meet the set of design constraints in eachscenario in the set of scenarios. The system can then optimize thecircuit design over the subset of the set of scenarios. In someembodiments, the system can update circuit information only in thesubset of the set of scenarios whenever a gate is changed in the circuitdesign during optimization, thereby substantially reducing the amount ofcomputation required to optimize the circuit design. Note that a subsetof a set can include all or a portion of the set.

In some embodiments, the system can determine the subset of the set ofscenarios by removing a first scenario from the set of scenarios if thefirst scenario and a second scenario in the set of scenarios areassociated with the same mode and a first corner associated with thefirst scenario is dominated by a second corner associated with thesecond scenario. Note that the second corner dominates the first cornerif the delay and output transition time values in the second corner aregreater than or equal to the corresponding delay and output transitiontime values in the first corner over different sized implementations ofa gate type, input transition times, and capacitive load values. Theterm “gate type” can refer to a family of gates that performs aparticular logical function (e.g., AND, OR, NAND, etc.). A librarytypically includes different sized implementations of each gate type.Each cell in the target technology library that is being used tooptimize the circuit design is typically an implementation of a gatetype.

In some embodiments, the system can determine the dominance relation onthe set of corners as follows. The system can represent the set ofcorners by a set of points in an n-dimensional space, wherein eachcorner in the set of corners is associated with a point in the set ofpoints, and wherein each coordinate of a point is associated with thevalue of a parameter in the corresponding corner. Next, the system canformulate a direct dominance problem on the set of points. The systemcan then determine the dominance relation on the set of corners bysolving the direct dominance problem.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates various steps in the design and fabrication of anintegrated circuit.

FIG. 2 illustrates delay curves for different corners in accordance withsome embodiments of the present invention.

FIG. 3A illustrates a dominance relation on a set of corners inaccordance with some embodiments of the present invention.

FIG. 3B illustrates a dominance relation on a set of corners inaccordance with some embodiments of the present invention.

FIG. 4 illustrates how a dominance relation on a set of corners can beused to perform scenario reduction in accordance with some embodimentsof the present invention.

FIG. 5 presents a flowchart that illustrates a process for using libraryanalysis to determine a dominance relation on a set of corners inaccordance with some embodiments of the present invention.

FIG. 6 presents a flowchart that illustrates a process for optimizing acircuit design over a set of scenarios in accordance with someembodiments of the present invention.

FIG. 7 illustrates a computer system in accordance with some embodimentsof the present invention.

FIG. 8 illustrates an apparatus in accordance with some embodiments ofthe present invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

FIG. 1 illustrates various steps in the design and fabrication of anintegrated circuit. The process starts with a product idea 100, which isrealized using Electronic Design Automation (EDA) software 110. Chips170 can be produced from the finalized design by performing fabrication150 and packaging and assembly 160 steps.

A design flow that uses EDA software 110 is described below. Note thatthe design flow description is for illustration purposes only, and isnot intended to limit the present invention. For example, an actualintegrated circuit design may require a designer to perform the designflow steps in a different sequence than the sequence described below.

In the system design step 112, the designers can describe thefunctionality to implement. They can also perform what-if planning torefine the functionality and to check costs. Further, hardware-softwarearchitecture partitioning can occur at this step. In the logic designand functional verification step 114, a Hardware Description Language(HDL) design can be created and checked for functional accuracy.

In the synthesis and design step 116, the HDL code can be translated toa netlist, which can be optimized for the target technology. Further,tests can be designed and implemented to check the finished chips. Inthe netlist verification step 118, the netlist can be checked forcompliance with timing constraints and for correspondence with the HDLcode.

In the design planning step 120, an overall floor plan for the chip canbe constructed and analyzed for timing and top-level routing. Next, inthe physical implementation step 122, placement and routing can beperformed.

In the analysis and extraction step 124, the circuit functionality canbe verified at a transistor level. In the physical verification step126, the design can be checked to correct any functional, manufacturing,electrical, or lithographic issues.

In the resolution enhancement step 128, geometric manipulations can beperformed on the layout to improve manufacturability of the design.Finally, in the mask data preparation step 130, the design can betaped-out for production of masks to produce finished chips.

A circuit optimization technique that optimizes a circuit design over aset of scenarios must meet the functional and performance goals (timing,power, area, electrical design rules, etc.) for the circuit design forall the scenarios. Note that, as used in this disclosure, the terms“optimize,” “optimizing,” “optimization,” etc., relate to a process thattries to improve one or more metrics associated with the circuit design.These terms are not intended to imply that a local or a global optimumvalue is being determined. The metrics could include electrical designrules (e.g., maximum capacitance, maximum transition, maximum fanout),leakage power, area, and timing-related metrics (e.g., Worst NegativeSlack (WNS) and Total Negative Slack (TNS)).

Each scenario can be associated with a corner and a mode. For example, ascenario may be associated with a positive defocus process corner and anormal operating mode. If there are no design requirement violations inthis scenario, the chip is expected to behave as desired in its normaloperating mode, even if the chip was manufactured using aphotolithography process that is positively defocused.

Traditional approaches optimize each scenario independently andsequentially. These approaches suffer from poor quality of results andlong run-times in converging on an optimal solution for all scenarios.Very often, these techniques fail to converge on an optimal solutionbecause of “ping-pong” effects. For example, if an optimizingtransformation performed in one scenario cancels the effects of anoptimizing transformation performed in another scenario, the techniquemay “ping-pong” between these two scenarios, thereby preventing thesolution from converging.

Some approaches model all of the scenarios concurrently and constructoptimal solutions under the constraints of all scenarios. For example,these approaches evaluate the timing of the circuit by performing statictiming analysis for all scenarios, and update timing information for allscenarios when an optimization transformation is applied to a gate. Notethat the term “gate” as used in this disclosure can generally refer toany portion of the circuit design that performs a logical function andthat may have one or more inputs and one or more outputs. In theseapproaches, the system optimizes the circuit design based on theslack/timing, leakage power, and DRC information over all scenarios.Although these approaches are superior to approaches that optimize onescenario at a time, they incur additional run-time and memory overhead.This is because, in these approaches, whenever timing, power, and DRCinformation (e.g., electrical design rules) are updated, they have to beupdated across all scenarios. Note that the run-time and memory overheadincrease with the number of scenarios. Since the number of scenarios israpidly increasing as semiconductor geometries become smaller andcircuit designs become more complex, these approaches are quicklybecoming computationally impractical.

Some embodiments of the present invention overcome the run-time andmemory overhead of approaches that concurrently analyze and optimize acircuit design over a set of scenarios by identifying a subset ofscenarios that is sufficient for achieving optimal implementationresults. Once the subset of scenarios has been identified, the analysisand optimization is performed only over the subset of scenarios.Specifically, timing, power, and DRC analyses are performed only on thesubset of scenarios. All other scenarios that do not belong to thesubset are not considered during analysis and optimization, therebyreducing the run-time and memory requirements.

A circuit design is typically optimized for a number of constraints oroptimization criteria (e.g., timing, power, electrical design rules,etc.) in each of the scenarios for each relevant object (e.g., timingend-point, net, cell, design, etc.) in the design. The system typicallyuses a design library which provides the optimization process a set ofimplementations for each gate type. During optimization the system canchange the circuit design by replacing an implementation of a gate typein the circuit design with another implementation that is selected fromthe design library. Each gate type implementation in the design librarycan have different characteristics. Further, the characteristics of agate type implementation can vary from one corner to another.

FIG. 2 illustrates delay curves for different corners in accordance withsome embodiments of the present invention. Each delay curve representsthe variation of the gate delay against the capacitive load for a givenimplementation of a gate type, input transition value, and corner.

Plot 202 illustrates four delay curves that correspond to corners C1,C2, C3, and C4. Note that these delay curves are for a particular inputtransition value and a particular gate size (i.e., a particularimplementation of a gate type in a technology library). Similar delaycurves can be created for other input transition values andimplementations of the gate type.

In plot 202, the gate delay in corner C1 is greater than the gate delaysin corners C2, C3, and C4 across all capacitive load values. In otherwords, corner C1 dominates corners C2, C3, and C4. Consider a circuitdesign that consists of a single gate whose delay characteristics aregiven by plot 202. If this circuit design meets timing requirements incorner C1, then we can conclude that the circuit design will meet timingrequirements in corners C2, C3, and C4. Thus, instead of optimizing thiscircuit design over all four corners, we only need to optimize thecircuit design over corner C1.

Plot 204 illustrates delay curves for a different input transition valueand/or a different implementation of the gate type in the sametechnology library. Specifically, in plot 204, the gate delays incorners C1 and C2 are greater than the gate delays in corners C3 and C4across all capacitive load values. However, depending on the capacitiveload, the gate delay in corner C1 may or may not be greater than thegate delay in corner C2 (the delay curve for corner C2 has been shown asa dotted line to clearly distinguish it from the delay curve for cornerC1). In other words, corners C1 and C2 dominate corners C3 and C4. Thus,in this situation, we need to optimize the single gate circuit design incorners C1 and C2 to ensure that the circuit design will meet timingrequirements across all corners.

Corner Ci can be said to dominate corner Cj if the gate delay in cornerCi is greater than or equal to the gate delay in corner Cj across allimplementations of a gate type, input transitions, and capacitive loads.Similarly, corner Ci can be said to dominate Cj if leakage power values(or electrical design rule values) in Ci are greater than or equal tocorresponding leakage power values (or electrical design rule values) inCj over a set of gate implementations of a gate type.

The notion of dominance can be extended to multiple parameter values(e.g., gate delay, leakage power, electrical design rules, etc.).Specifically, corner Ci can be said to dominate corner Cj if, for eachparameter in a set of parameters, the parameter value in corner Ci isgreater than or equal to the parameter value in corner Cj across theentire optimization space. A dominance relation D can be defined on aset of corners such that Ci D Cj if and only if corner Ci dominatescorner Cj.

FIG. 3A illustrates a dominance relation on a set of corners inaccordance with some embodiments of the present invention.

The dominance relation shown in FIG. 3A corresponds to the gate delaysshown in plots 202 and 204. In FIG. 3A, the dominance relation isrepresented as a set of corner tuples, wherein the corner tuple (Ci, Cj)indicates that corner Ci dominates corner Cj. Note that corner C1 wouldhave dominated corner C2 if we had only considered the implementation ofthe gate type in plot 202. However, since in plot 204 (which correspondsto a different implementation of the gate type) the gate delay in cornerC1 is less than the gate delay in corner C2 for at least one data point,corner C1 does not dominate corner C2.

FIG. 3B illustrates a dominance relation on a set of corners inaccordance with some embodiments of the present invention.

The dominance relation shown in FIG. 3B corresponds to the gate delaysshown in plots 202 and 204. In FIG. 3B, the dominance relation isrepresented visually as a directed graph, wherein a directed edge (Ci,Cj) indicates that corner Ci dominates corner Cj. The representations inFIGS. 3A and 3B are not intended to be exhaustive or to limit thepresent invention to the forms disclosed. Accordingly, many differentrepresentations of a dominance relation will be apparent topractitioners skilled in the art.

FIG. 4 illustrates how a dominance relation on a set of corners can beused to perform scenario reduction in accordance with some embodimentsof the present invention.

Suppose a circuit design needs to be optimized over the set of scenarios{S1, S2, S3, S4, S5, S6, S7}. As shown in FIG. 4, each scenario is acombination of a mode from the set of modes {M1, M2, M3}, and a cornerfrom the set of corners {C1, C2, C3, C4}. Let us assume that thedominance relation on the set of corners {C1, C2, C3, C4} is the same asthose illustrated in FIG. 3A or 3B.

The dominance relation on the set of corners can be used to performscenario reduction as follows. First, the system can group the scenariosbased on the associated modes. For example, as shown in FIG. 4, thesystem can create three scenario groups G1, G2, and G3 by grouping thescenarios based on the associated modes. Next, the system can use thedominance relation on the set of corners to reduce the number ofscenarios in each group. For example, since corner C4 is dominated bycorners C1 and C2, the system can remove scenario S3 from group G1.Similarly, scenarios S5 and S7 can be removed from groups G2 and G3,respectively. The resulting subset of the set of scenarios consists ofscenarios S1, S2, S4, and S6. Note that scenario reduction based on adominance relation on the set of corners must be performed on a per-modebasis because different modes can have vastly different supply voltages,design constraints, and clock speeds.

The scenario reduction process illustrated in FIG. 4 is for illustrationpurposes only and is not intended to limit the scope of the presentinvention. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. For example, in someembodiments, the system can use the dominance relation on the set ofcorners to determine a dominance relation on the set of scenarios.Specifically, scenario Si dominates scenario Sj if both Si and Sj areassociated with the same mode, and the corner associated with scenarioSi dominates the corner associated with scenario Sj. Once a dominancerelation on the set of scenarios is determined, the system can determinea dominant subset of scenarios. Let Σ be the set of scenarios. Adominant subset of scenarios, Δ, is a subset of Σ such that everyscenario in the set Σ-Δ is dominated by at least one scenario in Δ.

FIG. 5 presents a flowchart that illustrates a process for using libraryanalysis to determine a dominance relation on a set of corners inaccordance with some embodiments of the present invention.

The process can begin by receiving a design library which specifies gatecharacteristics at each corner in a set of corners (step 502). Thesystem can then use the design library to determine a dominance relationon the set of corners (step 504). Specifically, in some embodiments, thesystem can represent the set of corners by a set of points in ann-dimensional space, wherein each corner in the set of corners isassociated with a point in the set of points, and wherein eachcoordinate of a point is associated with the value of a parameter in thecorresponding corner. Next, the system can formulate a direct dominanceproblem on the set of points, and determine the dominance relation onthe set of corners by solving the direct dominance problem. Note that adominance relation on the set of corners can be determined at any levelof granularity. Specifically, a dominance relation can be determined foreach gate type and/or for the entire design library. A dominancerelation can be used to determine a dominant subset of corners.Specifically, let Σ be the set of corners. A dominant subset of corners,Δ, is a subset of Σ such that every corner in the set Σ-Δ is dominatedby at least one corner in Δ.

Next, the system can store the dominance relation in a non-transitorycomputer-readable storage medium (step 506). In some embodiments, thedominance relation can be stored with the design library.

FIG. 6 presents a flowchart that illustrates a process for performingscenario reduction using a dominance relation on a set of corners inaccordance with some embodiments of the present invention.

The process can begin by determining a subset of the set of scenariosusing a dominance relation on the set of corners (step 602). Note that,if the circuit design meets a set of design constraints in each scenarioin the subset of the set of scenarios, the circuit design is expected tomeet the set of design constraints in each scenario in the set ofscenarios. Specifically, the system can determine a subset of the set ofscenarios by removing a first scenario from the set of scenarios if thefirst scenario and a second scenario in the set of scenarios areassociated with the same mode and a first corner associated with thefirst scenario is dominated by a second corner associated with thesecond scenario.

The system can then optimize the circuit design over the subset of theset of scenarios (step 604). Specifically, the system can update circuitinformation only in the subset of the set of scenarios whenever a gateis changed in the circuit design during optimization, therebysubstantially reducing the amount of computation required to optimizethe circuit design over the set of scenarios.

Note that a design library can include a large number of gate types. Asexplained above, the system can determine a dominant subset of cornersfor each gate type in the design library. In some embodiments, thesystem can consider dominant subsets of corners associated with all gatetypes regardless of which gate types are expected to be used duringoptimization. Alternatively, the system can first identify the gatetypes that are expected to be used during optimization, and then useonly those dominant subsets of corners that are associated with theidentified gate types.

FIG. 7 illustrates a computer system in accordance with some embodimentsof the present invention.

Computer system 702 can include processor 704, memory 706, and storagedevice 708. Computer system 702 can be coupled to display device 714,keyboard 710, and pointing device 712. Storage device 708 can storeoperating system 718, applications 716, and data 720.

Applications 716 can include instructions which, when executed bycomputer system 702, can cause computer system 702 to perform processeswhich are inherently or explicitly described in this disclosure. Data720 can include any data that is required as input or that is generatedas output by processes which are inherently or explicitly described inthis disclosure.

FIG. 8 illustrates an apparatus in accordance with some embodiments ofthe present invention.

Apparatus 802 can comprise a plurality of mechanisms which maycommunicate with one another via a wired or wireless communicationchannel. Apparatus 802 may be realized using one or more integratedcircuits, and apparatus 802 may include fewer or more mechanisms thanthose shown in FIG. 8. Further, apparatus 802 may be integrated in acomputer system, or it may be realized as a separate device which iscapable of communicating with other computer systems and/or devices.

Apparatus 802 can include one or more mechanisms which are configured toperform operations which are inherently or explicitly described in thisdisclosure. Specifically, apparatus 802 can include receiving mechanism804, determining mechanism 806, optimizing mechanism 808, and storingmechanism 810.

In some embodiments, receiving mechanism 804 can be configured toreceive a set of scenarios, wherein each scenario is a combination of amode from a set of modes and a corner from a set of corners. Determiningmechanism 806 can be configured to determine a subset of the set ofscenarios using a dominance relation on the set of corners. Optimizingmechanism 808 can be configured to optimize a circuit design over thesubset of the set of scenarios.

In some embodiments, receiving mechanism 804 can be configured toreceive a design library that specifies circuit characteristics over aset of corners, determining mechanism 806 can be configured to determinea dominance relation on the set of corners, and storing mechanism 810can be configured to store the dominance relation on the set of cornersin a non-transitory computer-readable storage medium.

CONCLUSION

The above description is presented to enable any person skilled in theart to make and use the embodiments. Various modifications to thedisclosed embodiments will be readily apparent to those skilled in theart, and the general principles defined herein are applicable to otherembodiments and applications without departing from the spirit and scopeof the present disclosure. Thus, the present invention is not limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein.

The data structures and code described in this disclosure can bepartially or fully stored on a computer-readable storage medium and/or ahardware module and/or hardware apparatus. A computer-readable storagemedium includes, but is not limited to, volatile memory, non-volatilememory, magnetic and optical storage devices such as disk drives,magnetic tape, CDs (compact discs), DVDs (digital versatile discs ordigital video discs), or other media, now known or later developed, thatare capable of storing code and/or data. Hardware modules or apparatusesdescribed in this disclosure include, but are not limited to,application-specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), dedicated or shared processors, and/or otherhardware modules or apparatuses now known or later developed.

The methods and processes described in this disclosure can be partiallyor fully embodied as code and/or data stored in a computer-readablestorage medium or device, so that when a computer system reads andexecutes the code and/or data, the computer system performs theassociated methods and processes. The methods and processes can also bepartially or fully embodied in hardware modules or apparatuses, so thatwhen the hardware modules or apparatuses are activated, they perform theassociated methods and processes. Note that the methods and processescan be embodied using a combination of code, data, and hardware modulesor apparatuses.

The foregoing descriptions of embodiments of the present invention havebeen presented only for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

1. A method for optimizing a circuit design over a set of scenarios,wherein each scenario is associated with a corner from a set of cornersand a mode from a set of modes, the method comprising: determining asubset of the set of scenarios using a dominance relation on the set ofcorners such that if the circuit design meets a set of designconstraints in each scenario in the subset of the set of scenarios, thecircuit design is expected to meet the set of design constraints in eachscenario in the set of scenarios; and optimizing the circuit design overthe subset of the set of scenarios.
 2. The method of claim 1, whereindetermining the subset of the set of scenarios using the dominancerelation on the set of corners includes removing a first scenario fromthe set of scenarios if the first scenario and a second scenario in theset of scenarios are associated with the same mode and a first cornerassociated with the first scenario is dominated by a second cornerassociated with the second scenario.
 3. The method of claim 1, wherein asecond corner dominates a first corner if delay and output transitiontime values in the second corner are greater than or equal tocorresponding delay and output transition time values in the firstcorner over a set of implementations of a gate type, input transitiontimes, and capacitive load values.
 4. The method of claim 1, wherein asecond corner dominates a first corner if leakage power values in thesecond corner are greater than or equal to corresponding leakage powervalues in the first corner over a set of gate implementations of a gatetype.
 5. The method of claim 1, wherein a second corner dominates afirst corner if electrical design rule values in the second corner aregreater than or equal to corresponding electrical design rule values inthe first corner over a set of gate implementations of a gate type. 6.The method of claim 1, further comprising: representing the set ofcorners by a set of points in an n-dimensional space, wherein eachcorner in the set of corners is associated with a point in the set ofpoints, and wherein each coordinate of a point is associated with thevalue of a parameter in the corresponding corner; formulating a directdominance problem on the set of points; and determining the dominancerelation on the set of corners by solving the direct dominance problem.7. The method of claim 1, wherein circuit information is updated only inthe subset of the set of scenarios whenever a gate is changed in thecircuit design during optimization.
 8. The method of claim 1, whereinthe subset of the set of scenarios is determined using dominant subsetsof corners associated with gate types in a library.
 9. The method ofclaim 1, wherein determining the subset of the set of scenarios usingthe dominance relation on the set of corners includes: determining gatetypes that are expected to be used during optimization; and determiningthe subset of the set of scenarios using dominant subsets of cornersassociated with the gate types.
 10. A non-transitory computer-readablestorage medium storing instructions that when executed by a computercause the computer to perform a method for optimizing a circuit designover a set of scenarios, wherein each scenario is associated with acorner from a set of corners and a mode from a set of modes, the methodcomprising: determining a subset of the set of scenarios using adominance relation on the set of corners such that if the circuit designmeets a set of design constraints in each scenario in the subset of theset of scenarios, the circuit design is expected to meet the set ofdesign constraints in each scenario in the set of scenarios; andoptimizing the circuit design over the subset of the set of scenarios.11. The non-transitory computer-readable storage medium of claim 10,wherein determining the subset of the set of scenarios using thedominance relation on the set of corners includes removing a firstscenario from the set of scenarios if the first scenario and a secondscenario in the set of scenarios are associated with the same mode and afirst corner associated with the first scenario is dominated by a secondcorner associated with the second scenario.
 12. The non-transitorycomputer-readable storage medium of claim 10, wherein a second cornerdominates a first corner if the delay and output transition time valuesin the second corner are greater than or equal to the correspondingdelay and output transition time values in the first corner over a setof implementations of a gate type, input transition times, andcapacitive load values.
 13. The non-transitory computer-readable storagemedium of claim 10, wherein a second corner dominates a first corner ifleakage power values in the second corner are greater than or equal tocorresponding leakage power values in the first corner over a set ofgate implementations of a gate type.
 14. The non-transitorycomputer-readable storage medium of claim 10, wherein a second cornerdominates a first corner if electrical design rule values in the secondcorner are greater than or equal to corresponding electrical design rulevalues in the first corner over a set of gate implementations of a gatetype.
 15. The non-transitory computer-readable storage medium of claim10, further comprising: representing the set of corners by a set ofpoints in an n-dimensional space, wherein each corner in the set ofcorners is associated with a point in the set of points, and whereineach coordinate of a point is associated with the value of a parameterin the corresponding corner; formulating a direct dominance problem onthe set of points; and determining the dominance relation on the set ofcorners by solving the direct dominance problem.
 16. The non-transitorycomputer-readable storage medium of claim 10, wherein circuitinformation is updated only in the subset of the set of scenarioswhenever a gate is changed in the circuit design during optimization.17. The non-transitory computer-readable storage medium of claim 10,wherein the subset of the set of scenarios is determined using dominantsubsets of corners associated with gate types in a library.
 18. Thenon-transitory computer-readable storage medium of claim 10, whereindetermining the subset of the set of scenarios using the dominancerelation on the set of corners includes: determining gate types that areexpected to be used during optimization; and determining the subset ofthe set of scenarios using dominant subsets of corners associated withthe gate types.
 19. An apparatus for optimizing a circuit design over aset of scenarios, the apparatus comprising: a determining mechanismconfigured to determine a subset of the set of scenarios using adominance relation on the set of corners such that if the circuit designmeets a set of design constraints in each scenario in the subset of theset of scenarios, the circuit design is expected to meet the set ofdesign constraints in each scenario in the set of scenarios; and anoptimizing mechanism configured to optimize the circuit design over thesubset of the set of scenarios.
 20. The apparatus of claim 19, whereinthe determining mechanism is configured to remove a first scenario fromthe set of scenarios if the first scenario and a second scenario in theset of scenarios are associated with the same mode and a first cornerassociated with the first scenario is dominated by a second cornerassociated with the second scenario.
 21. The apparatus of claim 19,wherein a second corner dominates a first corner if delay and outputtransition time values in the second corner are greater than or equal tocorresponding delay and output transition time values in the firstcorner over a set of implementations of a gate type, input transitiontimes, and capacitive load values.
 22. The apparatus of claim 19,wherein a second corner dominates a first corner if leakage power valuesin the second corner are greater than or equal to corresponding leakagepower values in the first corner over a set of gate implementations of agate type.
 23. The apparatus of claim 19, wherein a second cornerdominates a first corner if electrical design rule values in the secondcorner are greater than or equal to corresponding electrical design rulevalues in the first corner over a set of gate implementations of a gatetype.
 24. The apparatus of claim 19, further comprising: a representingmechanism configured to represent the set of corners by a set of pointsin an n-dimensional space, wherein each corner in the set of corners isassociated with a point in the set of points, and wherein eachcoordinate of a point is associated with the value of a parameter in thecorresponding corner; a formulating mechanism configured to formulate adirect dominance problem on the set of points; and a second determiningmechanism configured to determine the dominance relation on the set ofcorners by solving the direct dominance problem.
 25. The apparatus ofclaim 19, wherein the optimizing mechanism is configured to updatecircuit information only in the subset of the set of scenarios whenevera gate is changed in the circuit design during optimization.
 26. Theapparatus of claim 19, wherein the determining mechanism is configuredto determine the subset of the set of scenarios using dominant subsetsof corners associated with gate types in a library.
 27. The apparatus ofclaim 19, wherein the determining mechanism is configured to: determinegate types that are expected to be used during optimization; anddetermine the subset of the set of scenarios using dominant subsets ofcorners associated with the gate types.